With regard to power converters, there is a carrier comparison method as one of representative methods of calculating a switching pattern to PWM control a semiconductor switching element constituting an inverter. According to the carrier comparison method, a voltage command to the inverter and a size of a carrier wave are compared, thereby determining an on/off pattern to perform a switch control. A triangular wave is often used as the carrier wave.
In a power converter using the carrier comparison method, a frequency of a carrier wave (hereinafter, “carrier frequency”) is an important parameter to determine a characteristic of the power converter. For example, when a carrier frequency is set at a high level, there is an effect such that the accuracy and response characteristic of an inverter output voltage improve, and thus electromagnetic noise generated from a load is reduced. On the other hand, when the carrier frequency is set at a high level, there is a problem such that a switching loss of a semiconductor switching element increases, and thus electromagnetic noise also increases. Therefore, an appropriate carrier frequency needs to be set by matching the type of a load connected to the inverter and its operating condition.
In performing a current control of a load connected to the inverter, a high current-control response is achieved by shortening an updating cycle of an output voltage by increasing a carrier frequency. Meanwhile, when a current control is continued by keeping a high carrier frequency, a switching loss of the inverter increases. That is, high response of current control and suppression of a switching loss of the inverter are in a tradeoff relationship, and therefore the carrier frequency needs to be determined by taking one of these factors with priority.
As a technique of solving such a problem, for example, there is a technique disclosed in Patent Document 1 mentioned below. Patent Document 1 discloses a technique of changing a carrier frequency according to a deviation (a control deviation) between a current command and a detected current. Specifically, when a control deviation is large, a carrier frequency is increased to improve its response characteristic. On the other hand, when the control deviation is small or none, the carrier frequency is decreased based on an idea that the control is being performed satisfactorily. That is, this method has a technical concept of increasing the carrier frequency only when a control response is necessary. Patent Document 1 also discloses a technique of changing a carrier frequency corresponding to a change rate of a current deviation based on a change rate of a detected current and a change rate of a current command, and has the same object as that of the above technique.
Patent Document 1: Japanese Patent Application Laid-open No. 2001-37248